acme looks like acme; every theme gets acme's button, scroll column and 2px rules

acme takes plan9port's own colours (acme.c iconinit, draw.h): selections
#eeee9e in the body and #9eeeee in the tag, black ink; exec and look sweeps
#aa0000 and #006600, white ink; a #99994c scrollbar under a #ffffea thumb;
black rules; a #8888cc tag/body rule and column button.

Every theme: the grip is acme's button, its tag row's full height, in one
scroll column with the scrollbar (rail_px, 12 at a 17px tagline, scaled,
times GripWidth, a percent, default 150; same x0/x1 to the pixel):
focused clean = solid box; focused dirty = box_dirty filled to a box ring
(acme's modbutton); unfocused clean = a box_border ring round the tag's
ground; unfocused dirty = box_dirty a pixel inside that ring. The grid, which
has no ring, marks dirty with a bold * on box_dirty in the grip's second cell
and fills an unfocused grip halfway to its ring. Regions carry a dirty flag.
Column buttons are the same shape, solid. 2px rules between columns and
stacked panes (inside the tag band's slack), none at the window edge; a 1px
tag/body rule halfway between tag and page.

Theme data: tag_sel_bg, sweep_bg/fg, tag_rule, rule_px, rail_px, box_border,
box_dirty. Chrome: rule_px, tag_rule, rail_px, grip_border, grip_focus_ring;
a comptime guard on Chrome's layout forces a wire bump once v8 ships.
Structural rules are exempt from the focus-indicator rule (docs/effects.md).

Tests: acme widths; stacked rule in the slack at two tagline sizes; every
grip-column pixel is band, button, ring or rule, and the button shares the
scrollbar's x0/x1 (4 metrics x top/bottom x focus x dirty); the four grip
states distinct in every native theme with a non-colour dirty cue in the
grid; wire round-trip of the new fields.

Shared files: src/colors.zig, src/surface.zig, src/body_layer.zig,
src/tagline.zig, src/detached/wire.zig, docs/themes.md, docs/config.md,
docs/effects.md, 21 snapshot goldens (the grip cells only),
test/gui-goldens.txt.

Motion flavours differ by design, and drive notice drops and pane moves

3 files changed, 16 insertions, 3 deletions

diff --git a/docs/config.md b/docs/config.md
index 83fbd0eb..324b54e9 100644
--- a/docs/config.md
+++ b/docs/config.md
@@ -315,8 +315,10 @@ name, thirteen required RGB roles (`sel_bg`, `sel_fg`, `tag_bg`, `tag_fg`,
 `scroll_thumb`), nullable `bg`/`fg`, and either a 16-color RGB `palette` or
 `null`. Optional nullable RGB roles extend this format:
 `tag_active_bg`, `tag_active_fg`, `tag_name_fg`, `tag_active_name_fg`, `border`, `lineno_active`, `search_bg`, `search_fg`,
-`diagnostic_error`, `diagnostic_warning`, `diagnostic_info`, and
-`diagnostic_hint`. Missing new roles use backward-compatible defaults, so
+`diagnostic_error`, `diagnostic_warning`, `diagnostic_info`,
+`diagnostic_hint`, `tag_sel_bg`, `tag_rule`, `box_border` and `box_dirty`;
+`sweep_bg` and `sweep_fg` take three triples, and `rule_px` and `rail_px` a
+pixel count. Missing new roles use backward-compatible defaults, so
 previously exported files remain valid. [Theme customization](themes.md)
 describes each role and its fallback. RGB values are three-byte arrays, and
 hex literals are accepted. The original fields remain required; there is no
@@ -536,6 +538,8 @@ Lift auto        a shadow on a light page; on a dark one the others recede
 Lift off
 InactiveDim 30   the unfocused panes' text fades 30% toward its ground
 Motion smooth    off, crisp, smooth, bouncy or playful
+GripWidth 150    the grip's button and the scrollbar under it, percent of the
+                 theme's rail_px (12px at a 17px tagline; 50 to 300)
 ```
 
 `InactiveDim` works everywhere (a grid shows it at once) and never takes a
diff --git a/docs/effects.md b/docs/effects.md
index fec2d13b..eefa52d3 100644
--- a/docs/effects.md
+++ b/docs/effects.md
@@ -6,7 +6,10 @@ selection) or reduce its contrast. The effects are sugar; the indicators are
 how a person knows where they are. Every G stage is reviewed against this.
 Lift, for one, falls only on pane bodies and rails, never on a tag, a grip or
 a header, and its strength is capped so text and the selection keep min(their
-contrast, 4.5) (tests in src/gui/gui.zig). A dim touches only the unfocused
+contrast, 4.5) (tests in src/gui/gui.zig). The rule binds effects, not a
+theme's structure: the rules between panes (2px, `rule_px`) are the theme's
+borders, and the one over a tag band stays in the band's slack, never over
+its text. A dim touches only the unfocused
 panes, with the same floor, and the focused pane's text is never at less
 contrast than theirs (tests in src/draw.zig).
 
diff --git a/docs/themes.md b/docs/themes.md
index 8cc4ff9b..6b218b13 100644
--- a/docs/themes.md
+++ b/docs/themes.md
@@ -114,6 +114,12 @@ so existing exported themes remain valid.
 | `diagnostic_warning` | Warning text | Same foreground fallback |
 | `diagnostic_info` | Informational diagnostic text | Same foreground fallback |
 | `diagnostic_hint` | Hint text | Same foreground fallback |
+| `tag_sel_bg` | A tag's selection ground | `sel_bg` |
+| `sweep_bg`, `sweep_fg` | Three triples each: the select, exec and look sweeps' ground and ink | `sel_bg` tinted toward `kw`, `str` and `num`, in `sel_fg` |
+| `tag_rule` | The one-pixel rule between a pane's tag and its body, quieter than the `rule_px` rules between panes and columns | Halfway between `tag_bg` and the page the shell draws |
+| `rule_px` | Pixel shells: the width of the rules between columns, between panes stacked in a column, and under the workspace and column tags, in `border` (the rule between a tag and its body is always one pixel) | 2 |
+| `box_border`, `box_dirty` | The grip is acme's button: `box` filled when focused, else a two-pixel ring of `box_border` round the tag's ground; `box_dirty` fills it inside either ring while its file is unsaved (a grid, which has no ring, shows a bold `*` on `box_dirty` in the grip's second cell) | `box_dim`; `diagnostic_warning`, then `num` |
+| `rail_px` | Pixel shells: the scroll column's width, the grip's button over the scrollbar (thumb a pixel narrower), in pixels at a 17px tagline, scaled with it | 12, acme's Scrollwid |
 
 Native palettes give filenames a distinct hue at approximately the same
 brightness as the surrounding tag text. This also applies to output names